DC-DC converter

ABSTRACT

A Single Ended Primary Inductance Converter (SEPIC) fed BUCK converter includes: a first switch configured to open or close according to a first signal; a SEPIC portion coupled to the first switch and coupled to an energy source, the SEPIC portion comprising a first set of one or more passive components; a BUCK converter portion coupled to the first switch, the BUCK converter portion comprising a second set of one or more passive components. While the first switch is closed, the SEPIC portion is configured to store energy from an energy source in at least some of the first set of passive components and deliver energy to the BUCK portion, and the BUCK converter portion is configured to deliver energy to a load and to store energy in at least some of the second set of passive components. While the first switch is open, the SEPIC portion is configured to deliver at least some of its stored energy to the load, and the BUCK converter portion is configured to deliver at least some of its stored energy to the load.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/992,194 entitled METHOD AND APPARATUS FOR POWER CONVERSION filedDec. 4, 2007 which is incorporated herein by reference for all purposes;and claims priority to U.S. Provisional Patent Application No.61/013,187 entitled METHOD AND APPARATUS FOR POWER CONVERSION filed Dec.12, 2007 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Modern electronic devices often require power conversion. For example,battery operated devices such as notebook computers and mobile phonesoften include microprocessors that require the batteries to supply lowvoltages and high currents. BUCK converter is a type of step-downconverter often used in DC-DC power conversion applications. FIG. 1 is aschematic diagram illustrating a conventional BUCK converter. BUCKconverter 100 is sometimes referred to as a synchronized BUCK converterbecause switches S_(1B) and S_(2B) are synchronized to alternately turnon or off.

Conversion efficiency and transient response are important parameters ofstep-down converters. Conversion efficiency determines how much power islost during power conversion; transient response determines how quicklythe converter can respond to load current or source voltage changes. Inthe conventional topology shown in FIG. 1, it is often difficult to bothincrease conversion efficiency and improve transient response sinceswitch and parasitic losses are directly proportional to the switch modefrequency, while the value of the integrating inductor L_(BUCK)determines the first order transient response and is inverselyproportional.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a schematic diagram illustrating a conventional BUCKconverter.

FIG. 2A is a schematic diagram illustrating an embodiment of a SEPIC FEDBUCK converter.

FIG. 2B is a diagram illustrating the magnetic structure of device 200of FIG. 2A, with attendant voltage, current, and SEPIC FED BUCK couplingidentities.

FIG. 2C is a set of graphs illustrating the timing, voltage, and currentidentities of device 200 of FIG. 2A, with attendant timing, voltage, andcurrent summation expressions.

FIG. 2D is a schematic diagram illustrating an embodiment of an SFBconverter that is configured to perform a Gate Charge Extraction (GCE)process when the S_(1SB) switch is turned off.

FIG. 2E is a schematic diagram illustrating an embodiment of acommutation matrix included in SFB converter 200 of FIG. 2A.

FIG. 3 is a graph illustrating the turn-on or turn-off loss ratios (K)associated with a S_(1B) switch of a conventional BUCK converter and aS_(1SB) switch of a comparatively identical SFB converter embodiment.

FIG. 4A is a graph illustrating the first order approximation of turn-onand turn-off losses associated with switch S_(1B) of BUCK converter 100.

FIG. 4B is a graph illustrating the first order approximation of turn-onand turn-off losses associated with switch S_(1SB) of SFB converter 250,as well as attendant switch voltage, switch current, and switch powerloss identities and expressions in terms of the duty cycle of the switch(D).

FIG. 5A is a schematic diagram illustrating an embodiment of a singlemagnetic, magnetically coupled SEPIC FED BUCK converter with attendantvoltage, current, and transfer function (M) identities.

FIG. 5B illustrates the magnetic structure of converter 500 of FIG. 5A,with attendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 5C is a set of graphs illustrating the timing, voltage, and currentidentities of device 500 of FIG. 5A, with attendant timing, voltage, andcurrent summation expressions.

FIG. 5D is a schematic diagram illustrating an SFB converter during aGCE process.

FIG. 5E is a schematic diagram illustrating a commutation matrixincluded in SFB converter 500 of FIG. 5A.

FIG. 6A is a schematic diagram illustrating an embodiment of amulti-phase magnetically coupled, single magnetic SFB converter withattendant voltage, current, and transfer function (M) identities.

FIG. 6B is a diagram illustrating the magnetic structure of converter600 of FIG. 6A, with attendant voltage, current, and SEPIC FED BUCKcoupling identities.

FIG. 6C is a set of graphs illustrating the timing, voltage, and currentidentities of device 600 of FIG. 6A, with attendant timing, voltage, andcurrent summation expressions.

FIG. 7A is a schematic diagram illustrating another embodiment of a SFBconverter.

FIG. 7B is a diagram illustrating the magnetic structure of SFBconverter 700 of FIG. 7A, with attendant voltage, current, and SEPIC FEDBUCK coupling identities.

FIG. 7C is a set of graphs illustrating the timing, voltage, and currentidentities of SFB converter 700 of FIG. 7A, with attendant timing,voltage, and current summation expressions.

FIGS. 8A-8D illustrate the inductive windings in a conventional BUCKconverter and in several SFB converters, with attendant currentidentities and dimensional expressions.

FIG. 8E is a graph illustrating the conductive loss ratios of aconventional BUCK converter and SFB converters.

FIG. 9 is a graph illustrating the inductor set/reset ratios of a SFBconverter (e.g., SFB 200, 500, 600 or 700) and a canonical BUCKconverter.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; and/or a composition of matter. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Embodiments of a Single Ended Primary Inductance Converter (SEPIC) fedBUCK (SFB) converter are disclosed. The converter includes a SEPICportion that is galvanically or magnetically coupled to a BUCK converterportion. The SEPIC portion and the BUCK converter portion share aswitch. While the switch is closed, the SEPIC portion is configured tostore energy from an energy source and to deliver energy to the BUCKconverter portion, and the BUCK converter portion is configured todeliver energy it receives from the SEPIC portion to the load and tostore energy. While the switch is open, the SEPIC portion is configuredto deliver at least some of its stored energy to the load, and the BUCKconverter portion is configured to deliver at least some of its storedenergy to the load.

FIG. 2A is a schematic diagram illustrating an embodiment of a SEPIC FEDBUCK converter. An ideal circuit without parasitic effects is shown forpurposes of clarity. In this example, device 200 includes a SEPICportion coupled to a BUCK converter portion. Switch S_(1SB) is coupledto both the SEPIC portion and the BUCK converter portion. As will bedescribed in greater detail below, the SEPIC portion and the BUCKconverter portion are galvanically coupled. The SEPIC portion includesswitch S_(2S) (also referred to as the SEPIC portion associated switch)and a set of passive components including coupled inductors T_(1A) andT_(1B), capacitor C₂, as well as optional input capacitor C₁. An energysource E_(in) (such as a battery) is coupled to the inductors at inputnodes A and E. The negative terminal of the energy source is sometimesreferred to as the ground terminal. The BUCK converter portion includesswitch S_(2B) (also referred to as the BUCK converter portion associatedswitch) and a set of passive components. In this case the passivecomponents include inductor T_(1C). A load R is optionally coupledbetween a terminal of inductor T_(1C) and the negative terminal of theinput source. Switch S_(1SB) is configured to open or close according toa first switching signal. Switches S_(2B) and S_(2S) are configured toopen or close according to a second switching signal. In the embodimentshown, the switching signals are provided by a controller 206, which isoptionally included in the SFB converter in some embodiments. In variousembodiments, the controller may be a discrete component separatelycoupled to the SFB converter circuitry, or an integrated component ofthe circuitry. The first and second switching signals are synchronizedto be opposite of each other. In other words, when S_(1SB) is open,S_(2B) and S_(2S) are closed, and vice versa. For purposes of clarity inthe following discussions it is assumed that in the converter circuit,the inductors have the same inductance. Different inductance values maybe used in other embodiments. The transfer function of the converter(i.e., the ratio of the output voltage E_(out) to the input voltageE_(in)) is expressed as:M=D/(2−D),where D is the duty cycle of the switching signal associated withS_(1SB), and where D={1−[(E_(in)−E_(out))/(E_(in)+E_(out))]}.

In the embodiment shown, an input capacitor C₁, an intermediatecapacitor C₂, an output capacitor C₃ are included to provide integratingfunctions. S_(2B) and S_(2S) are implemented using transistors thatinclude gate terminals. An optional drive inductor T_(1D) is included inthe circuit to provide common mode drive to the gate terminal of S_(2S)to turn the transistor of S_(2S) off or on, thereby opening or closingthe switch. T_(1D), however, does not substantially perform powerconversion function in this example. In some embodiments, drive inductorT_(1D) is replaced with a solid state driver or any other appropriatedriver.

FIG. 2B is a diagram illustrating the magnetic structure of device 200of FIG. 2A, with attendant voltage, current, and SEPIC FED BUCK couplingidentities. In this example, inductive windings T_(1A), T_(1B), T_(1C),and T_(1D) share the same magnetic core.

FIG. 2C is a set of graphs illustrating the timing, voltage, and currentidentities of device 200 of FIG. 2A, with attendant timing, voltage, andcurrent summation expressions. In the examples shown, T represents aperiod of the switching signal, t_(ON) represents the time period duringwhich switch S_(1SB) is closed (in other words, the transistor used toimplement the switch is turned on), and t_(OFF) represents the timeperiod during which switch S_(1SB) is open (the transistor is turnedoff). The duty cycle of the switching signal is represented as D.

Referring to FIGS. 2A and 2C, during t_(ON), S_(1SB) is closed whileS_(2B) and S_(2S) are open. DC currents I₁ and I₃ flow through inductorsT_(1A) and T_(1B), respectively. Thus, energy from the source is storedin the inductors in the SEPIC portion. During this time, the SEPICportion does not directly deliver energy to the load, but deliversenergy from the source to the BUCK converter portion. At the same time,current I₆ flows through T_(1C). Energy is therefore stored in theinductor in the BUCK converter portion. According to FIG. 2C, I₂ (graphJ) is the current through capacitor C₂, and I₉ (graph P) is the currentthrough capacitor C₃. During t_(ON), C₂ and C₃ discharge (set), andcurrent I_(out) is delivered to the load. Thus, the BUCK converterportion delivers energy to the load during t_(ON).

Again referring to FIGS. 2A and 2C, during t_(OFF), S_(1SB) is openwhile S_(2B) and S_(2S) are closed. Inductors T_(1A) and T_(1B) maintainDC current flow. The SEPIC portion delivers at least some of its storedenergy through switch S_(2S) to the load without substantially storingenergy in its inductors. Because the closed switch S_(2S) forms anelectrical path between the SEPIC portion and the load and because theelectrical path has DC continuity, the energy transfer process does notrequire transformer action. Thus, the circuit is said to be galvanicallycoupled. The BUCK converter portion also delivers at least some of theenergy that was stored in its inductor during t_(OFF). The BUCKconverter portion, however, does not substantially store energy duringt_(OFF). C₂ and C₃ charge (resest) during this period. The ON/OFF cycleis then repeated.

In some embodiments, the SFB converter is configured to implement a gateextraction process to reduce turn-off power loss and improve turn-offspeed. FIG. 2D is a schematic diagram illustrating an embodiment of anSFB converter that is configured to perform a Gate Charge Extraction(GCE) process when the S_(1SB) switch is turned off. In this example,device 250 is similar to device 200 shown in FIG. 2A. Switches S_(1SB),S_(2B) and S_(2S) are implemented using metal-oxide field-effecttransistors (MOSFETs). Outputs of drivers DR1 and DR2 are coupled to thegates of the MOSFETs, providing switching signals that turn thetransistors off and on. Driver return terminals 262 and 264 are coupledto the sources of their respective MOSFETs. During t_(OFF), the voltageapplied to the gate terminal of MOSFET S_(1SB) drops to turn the deviceoff. Inductor T_(1A), however, will maintain its current flow, thuscausing a current 268 to flow from the gate to the driver, therebyextracting charges accumulated in the gate-source capacitance of theMOSFET. Current 268 is therefore referred to as the GCE current. Sinceinductor T_(1B) is coupled to T_(1A), a current 270 is induced inT_(1B). Current 270, referred to as the GCE induced current, flows in aloop in the opposite direction as current 268. Currents 268 and 270combine to form a turn-off current. The GCE process allows SFB converter250 to have fast turn off time and low turn off loss.

In some embodiments, the SFB converter includes a commutation matrix toimprove the converter's turn-on characteristics by using a capacitanceset/reset process to contain parasitic energy. FIG. 2E is a schematicdiagram illustrating an embodiment of a commutation matrix included inSFB converter 200 of FIG. 2A. In the example shown, commutation matrix280 includes a set of diodes and capacitors. Nodes B, C, E, and F of thecommutation matrix are coupled to nodes B, C, E, and F of SFB converter200. Voltage identities associated with the capacitors C_(com1),C_(com2), and C_(com3) are expressed as:E _(Ccom1) =E _(Ccom2)=(E _(in) +E _(out))/2; andE _(Ccom3)=(E _(in) −E _(out))/2.

FIG. 3 is a graph illustrating the turn-on or turn-off loss ratios (K)associated with a S_(1B) switch of a conventional BUCK converter and aS_(1SB) switch of a comparatively identical SFB converter embodiment. Inthis example, turn-on or turn-off loss of switch S_(1SB) of SFBconverter 250 is compared with that of switch S_(1B) of conventionalBUCK converter 100. In this example, converters 100 and 250 are said tobe comparatively identical since they are assumed to have switches withidentical characteristics, and the same E_(in) and I_(out). The switchesare assumed to turn on and off at the same rate. When the switch isturned on (i.e., the switch is closed), the voltage across the switchdoes not drop to zero instantaneously, therefore causes turn-on loss.When the switch is turned off (i.e., the switch is open), the currentthrough the switch also does not drop to zero instantaneously and alsocauses turn-off loss. The turn-on and turn-off loss of the conventionalBUCK converter 100 is assumed to be 1, shown as line 300.

A first order approximation of the ratio of the turn-on loss of the SFBconverter 250 to the turn-on loss of the BUCK converter 100 is expressedas:K _(SFBon)=1/(2−D)³,where D is the duty cycle of the switching signal. The loss curve as afunction of D corresponds to curve 302 in the figure.

A first order approximation of the ratio of turn-off loss of the SFBconverter to turn-off loss of the BUCK converter is expressed as:K _(SFBoff) =a ²/[2E _(in) ²(2−D)],where a corresponds to a device transconductance characteristic andE_(in) corresponds to an input voltage of the converter. The loss curvecorresponds to curve 304.

A first order approximation of the ratio of the total (turn-on plusturn-off) loss of the SFB converter to the total loss of the BUCKconverter is expressed as:K _(SFBTotal)=0.5(K _(SFBon) +K _(SFBoff)).The loss curve corresponds to curve 306.

FIG. 4A is a graph illustrating the first order approximation of turn-onand turn-off losses associated with switch S_(1B) of BUCK converter 100.The attendant switch voltage, switch current, and switch poweridentities and expressions in terms of the duty cycle of the switch (D)are also illustrated.

FIG. 4B is a graph illustrating the first order approximation of turn-onand turn-off losses associated with switch S_(1SB) of SFB converter 250,as well as attendant switch voltage, switch current, and switch powerloss identities and expressions in terms of the duty cycle of the switch(D). Since the operating current I_(D) associated with switch S_(1SB) ofSFB converter 250 is significantly less than the operating current I_(D)associated with switch S_(1B) of BUCK converter 100, the turn-on loss issignificantly reduced. A first order approximation of turn-on power lossassociated with turning on S_(1SB) is:P _(LossSFBon)=[0.25(E _(in) +E _(out))·I _(out)(2−D)]·T _(turn-on) ·f,wherein E_(in) corresponds to the input voltage of the converter,I_(out) corresponds to the output current of the converter, Dcorresponds to the duty cycle of the switching signal, T_(turn-on)corresponds to the amount of time required to turn on the switch, and fcorresponds to the frequency of the switching signal.

A first order approximation of turn-off power loss associated withturning off S_(1SB) is:P _(LossSFBoff)=0.5a·[I _(out)/(2−D)]·T _(turn-off) ·f,wherein a corresponds to a device transconductance characteristic (whichequals 2 volts in this example), I_(out) corresponds to an outputcurrent of the converter, D corresponds to the duty cycle of theswitching signal, T_(turn-off) corresponds to the turn-off time of theswitch, and f corresponds to the frequency of the switching signal.

Several other SEPIC FED BUCK converter topologies exist. FIG. 5A is aschematic diagram illustrating an embodiment of a single magnetic,magnetically coupled SEPIC FED BUCK converter with attendant voltage,current, and transfer function (M) identities. Converter 500 shown inthis example includes a SEPIC portion and a BUCK converter portion thatare magnetically coupled. The portions are said to be magneticallycoupled because there is no galvanic path for transferring energy fromthe SEPIC portion to the load when S_(1SB) is turned off; instead,inductors T_(1C) and T_(1D) act as transformers to transfer energystored in SEPIC windings T_(1A) and T_(1B) to the load. FIG. 5Billustrates the magnetic structure of converter 500 of FIG. 5A, withattendant voltage, current, and SEPIC FED BUCK coupling identities.

FIG. 5C is a set of graphs illustrating the timing, voltage, and currentidentities of device 500 of FIG. 5A, with attendant timing, voltage, andcurrent summation expressions.

FIG. 5D is a schematic diagram illustrating an SFB converter during aGCE process. SFB converter 550 shown in this example is similar toconverter 500 of FIG. 5A. SFB converter 550 is magnetically coupled. Asshown in this diagram, when S_(1SB) switches off, GCE current 568 flowsin the opposite direction as GCE induced current 570, and charges in thegate-source capacitance of switch S_(1SB) are quickly removed.

FIG. 5E is a schematic diagram illustrating a commutation matrixincluded in SFB converter 500 of FIG. 5A. Nodes B, C, and E of thecommutation matrix are coupled to nodes B, C, E of SFB converter 500.Voltage identities associated with capacitors C_(com1) and C_(com2) areexpressed as:E _(Ccom1) =E _(Ccom2)=(E _(in) +E _(out))/2.

In some embodiments, the SFB is configured as a multi-phase converter.FIG. 6A is a schematic diagram illustrating an embodiment of amulti-phase magnetically coupled, single magnetic SFB converter withattendant voltage, current, and transfer function (M) identities. Inthis example, converter 600 includes a first SEPIC portion comprisinginductors T_(1A) and T_(1B) and switch S_(1S), and a second SEPICportion comprising inductors T_(1E) and T_(1F) and switch S_(2S). Theinductors SEPIC portions are magnetically coupled. The input and outputare isolated by a transformer comprising the inductive windings. Theconverter further includes a first BUCK converter portion comprisinginductors T_(1C) and T_(1G) and switch S_(1B), and a second BUCKconverter portion comprising inductors T_(1D) and T_(1H) and switchS_(2B). The inductors in the BUCK converter portions are alsomagnetically coupled. Switch S_(1SB) couples the first SEPIC portion tothe first BUCK converter portion, and switch S_(2SB) couples the secondSEPIC portion to the second BUCK converter portion. A commutation matrixsimilar to what was shown in FIG. 5E is included in the converter.

FIG. 6B is a diagram illustrating the magnetic structure of converter600 of FIG. 6A, with attendant voltage, current, and SEPIC FED BUCKcoupling identities.

FIG. 6C is a set of graphs illustrating the timing, voltage, and currentidentities of device 600 of FIG. 6A, with attendant timing, voltage, andcurrent summation expressions. The switching signals for switchesS_(1SB) and S_(2SB) have a phase offset. The switching signals forswitches S_(1B) and S_(1SB) are opposite of each other, and theswitching signals for switches S_(2B) and S_(2SB) are opposite. A firstswitching signal controlling switches S_(1SB), S_(1S) and S_(1B) have aphase offset relative to a second switching signal controlling switchesS_(2SB), S_(2S) and S_(2B). The first switching signal causes switchesS_(1SB), S_(1S) and S_(1B) to operate in concert such that when S_(1SB)is closed, the first SEPIC portion stores energy, and the first BUCKconverter portion delivers energy to the load and stores energy; whenS_(1SB) is open, the first SEPIC portion and the first BUCK converterportion both deliver energy to the load. The second switching signalcauses switches S_(2SB), S_(2S) and S_(2B) to similarly affect theoperations of the second SEPIC portion and the second BUCK converterportion.

Although the above example shows a 2 phase isolated SFB converter, someconverter embodiments are configured to include additional SEPIC andBUCK converter portions coupled in a similar manner to produce anN-phase SFB converter.

FIG. 7A is a schematic diagram illustrating another embodiment of a SFBconverter. In this example, SFB converter 700 is magnetically coupled.In various embodiments, T_(1C) and T_(1D) may be combined into a singleconductor or separated as multiple conductors. FIG. 7B is a diagramillustrating the magnetic structure of SFB converter 700 of FIG. 7A,with attendant voltage, current, and SEPIC FED BUCK coupling identities.A commutation matrix similar to FIG. 5E is optionally coupled to theconverter at nodes B, C, and E. FIG. 7C is a set of graphs illustratingthe timing, voltage, and current identities of SFB converter 700 of FIG.7A, with attendant timing, voltage, and current summation expressions.As shown in current I₂ (graph J), one of the SEPIC inductors T_(1B)principally conducts current during the GCE process. Thus, SFB converter700 experiences turn-off energy loss that is even smaller than SFBconverter embodiments 200 and 500.

Compared to conventional BUCK converters, SFB converters have reducedconductive loss because of the way the inductive windings are deployedin SFB converters. FIGS. 8A-8D illustrate the inductive windings in aconventional BUCK converter and in several SFB converters, withattendant current identities and dimensional expressions. In FIG. 8A,four inductive windings T_(1A), T_(1B), T_(1C), and T_(1D) of BUCKconverter 100 are shown. The inductive windings share the same magneticcore. The same magnetic windings are also present in FIG. 8B, FIG. 8C,and FIG. 8D, which correspond to SFB converter 200, 500, and 700,respectively. The windings of the BUCK converter and the SFB convertersare dimensionally identical since they have the same magnetic core area,window area, and number of turns. Different converter topologies,however, result in different amounts of current through individualwindings. Assuming that the converters have the same output power andinclude windings that have the same resistance, the amounts of energydissipated in the windings are different since the current values aredifferent.

FIG. 8E is a graph illustrating the conductive loss ratios of aconventional BUCK converter and SFB converters. The graph compares theloss ratios of the conventional BUCK converter 100 and the SFBconverters 200, 500, and 700. It is assumed that the converters havediscrete components of the same values and have the same E_(in) andI_(out). The switches are assumed to turn on and off at the same rate.The conductive loss ratio (K) is expressed in terms of duty cycle (D).The conductive loss ratio of BUCK converter 100 is assumed to be 1,shown as line 800.

The conductive loss ratio of SFB 200 is shown as curve 802 and isexpressed as:K=2(1−D+D ²)/(2−D)²

The conductive loss ratios of SFB 500 and 700 are the same. The ratio asa function of D is shown as curve 804, and is expressed as:K=(2−0.5D)/(2−D)².

The SFB converters also have faster transient response attributes. Incomparison with a comparative identical conventional BUCK converter, thetransient EMF (set) volt second (Et) of the integrating inductor and theMMF (reset) volt second (Et) of the integrating inductor in the SFBconverter are both lower. FIG. 9 is a graph illustrating the inductorset/reset ratios of a SFB converter (e.g., SFB 200, 500, 600 or 700) anda canonical BUCK converter. The ratio K is expressed in terms oftransfer function M. Assuming that the set ratio and the reset ratio ofthe conventional BUCK converter 100 are both 1, which is shown as line900. The reset ratio of a SFB converter with the same passive componentvalues, output, and switching duty cycle is shown as line 902 and isexpressed as:K _(SFBreset)=[(1+M)/2]²,where M=D/(2−D).

The set ratio of the SFB converter is shown as line 904 and is expressedas:K _(SFBSet)=(1+M)/2.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

1. A DC-DC converter comprising: a switch S_(1SB), a switch S_(2B), aswitch S_(2S), a capacitor C₂, an inductor T_(1A), an inductor T_(1B),and an inductor T_(1C); wherein: a first terminal of inductor T_(1A) iscoupled to a first terminal of an energy source; a first terminal ofcapacitor C₂, a second terminal of inductor T_(1A), and a first terminalof switch S_(1SB) are coupled; a first terminal of inductor T_(1C), asecond terminal of S_(1SB), and a first terminal of S_(2B) are coupled;a second terminal of inductor T_(1C) and a first terminal of S_(2S) arecoupled; a first terminal of inductor T_(1B) and a second terminal ofS_(2B) are configured for coupling with a second terminal of the energysource; and a second terminal of capacitor C₂, a second terminal ofinductor T_(1B), and a second terminal of S_(2S) are coupled.
 2. TheDC-DC converter of claim 1, further comprising a capacitor C1 and acapacitor C3; wherein: a first terminal of capacitor C₁ is coupled tothe first terminal of inductor T_(1A) and a second terminal of capacitorC1 is coupled to the first terminal of inductor T_(1B); and a firstterminal of capacitor C₃ is coupled to the second terminal of inductorT_(1C) and a second terminal of capacitor C₃ is coupled to the firstterminal of inductor T_(1B).
 3. The DC-DC converter of claim 1, whereininductors T_(1A), T_(1B), and T_(1C) are inductively coupled.
 4. TheDC-DC converter of claim 1, wherein the second terminal of inductorT_(1C) is coupled to a load.
 5. The DC-DC converter of claim 1, furthercomprising a commutation matrix coupled to the second terminal ofinductor T_(1A), the first terminal of inductor T_(1C), and the firstterminal of T_(1B), and the second terminal of inductor T_(1B).
 6. TheDC-DC converter of claim 1, further comprising an inductor T_(1D),wherein the first terminal of inductor T_(1D) is coupled to a gateterminal of switch S_(2B) and a second terminal of inductor T_(1D) iscoupled to a gate terminal of switch S_(2S).
 7. The DC-DC converter ofclaim 1, further comprising a controller configured to provide a signalto switch S_(1SB).
 8. A DC-DC converter comprising: a switch S_(1SB), aswitch S_(2B), a switch S_(2S), a capacitor C₂, an inductor T_(1A), aninductor T_(1B), an inductor T_(1C), an inductor T_(1D), and wherein: afirst terminal of inductor T_(1A) is coupled to a first terminal of anenergy source; a second terminal of inductor T_(1A), the first terminalof inductor T_(1B) and a first terminal of switch S_(1SB) are coupled;the first terminal of inductor T_(1C), a first terminal of inductorT_(1D), a second terminal of switch S_(1SB), a first terminal of switchS_(2B), and a first terminal of switch S_(2S) are coupled; a secondterminal of inductor T_(1C) and a second terminal of inductor T_(1D) arecoupled; and a second terminal of capacitor C₂, a second terminal ofswitch S_(2B), and a second terminal of switch S_(2S) are coupled. 9.The DC-DC converter of claim 8, further comprising a capacitor C₁ and acapacitor C₃; wherein: a first terminal of capacitor C₁ is coupled tothe first terminal of inductor T_(1A) and a second terminal of capacitorC₁ is coupled to the second terminal of capacitor C₂; and a firstterminal of capacitor C₃ is coupled the second terminal of inductorT_(1C) and a second terminal of capacitor C₃ is coupled to the secondterminal of capacitor C₂.
 10. The DC-DC converter of claim 8, whereininductors T_(1A), T_(1B), T_(1C), and T_(1D) are inductively coupled.11. The DC-DC converter of claim 8, wherein the second terminal ofinductor T_(1C) and the second terminal of T_(1D) are coupled to a load.12. The DC-DC converter of claim 8, further comprising a commutationmatrix coupled to the second terminal of inductor T_(1B), the firstterminal of inductor T_(1C), and the second terminal of capacitor C₂.13. The DC-DC converter of claim 8, further comprising a controllerconfigured to provide a signal to switch S_(1SB).
 14. A DC-DC convertercomprising: an inductor T_(1A), an inductor T_(1B), an inductor T_(1C),an inductor T_(1D), an inductor T_(1E), an inductor T_(1F), an inductorT_(1G), an inductor T_(1H), a capacitor C₂, a switch S_(1SB), a switchS_(2SB), a switch S_(1S), a switch S_(2S), a switch S_(1B), and a switchS_(2B); wherein: a first terminal of inductor T_(1A), a first terminalof inductor T_(1F), and a first terminal of an energy source arecoupled; a second terminal of inductor T_(1A), a first terminal ofinductor T_(1B), and a first terminal of switch S_(1SB) are coupled; asecond terminal of switch S_(1SB), a first terminal of switch S_(1S),and a first terminal of inductor T_(1C) are coupled; a second terminalof inductor T_(1C) and a first terminal of inductor T_(1D) are coupled;a first terminal of capacitor C₂, a second terminal of switch S_(1S),and a first terminal of S_(2S) are coupled; a second terminal ofinductor T_(1D), a first terminal of S_(2SB), and a second terminal ofS_(2S) are coupled; a second terminal of inductor T_(1E) and a secondterminal of inductor T_(1F) are coupled; a first terminal of switchS_(1B) and a first terminal of inductor T_(1G) are coupled; a firstterminal of switch S_(2B) and a first terminal of inductor T_(1H) arecoupled; a second terminal of switch S_(1B) and a second terminal ofswitch S_(2B) are coupled to a first output terminal; a second terminalof inductor T_(1G) and a second terminal of inductor T_(1H) are coupledto a second output terminal; and inductor T_(1C), inductor T_(1D),inductor T_(1G), and inductor T_(1H) are magnetically coupled.
 15. TheDC-DC converter of claim 14, further comprising a capacitor C₁ and acapacitor C₃; wherein: a first terminal of capacitor C₁ is coupled thefirst terminal of inductor T_(1A) and the second terminal of capacitorC₁ is coupled to the ground terminal; and the first terminal of inductorC₃ is coupled to the first output terminal and the second terminal ofinductor C₃ is coupled to the second output terminal.
 16. The DC-DCconverter of claim 14, wherein inductors T_(1A) and T_(1B) areinductively coupled.
 17. The DC-DC converter of claim 14, wherein thefirst output terminal and the second output terminals are coupled to aload.
 18. The DC-DC converter of claim 14, further comprising acommutation matrix coupled to node B, node C, and node E.
 19. The DC-DCconverter of claim 14, further comprising a controller configured toprovide a signal to switch S_(1SB).
 20. A DC-DC converter, comprising: aswitch S_(1SB), a switch S_(2B), a capacitor C₂, an inductor T_(1A), aninductor T_(1B), and an inductor T_(1C); wherein: a first terminal ofinductor T_(1A) is coupled to a first terminal of an energy source; asecond terminal of inductor T_(1A), a first terminal of inductor T_(1B),and a first terminal of switch S_(1SB) are coupled; a second terminal ofswitch S_(1SB), a first terminal of inductor T_(1C), and a firstterminal of switch S_(2B) are coupled; a second terminal of inductorT_(1C) is coupled to a first output terminal; a first terminal ofcapacitor C₂ and a second terminal of inductor T_(1B) are coupled; asecond terminal of capacitor C₂, a second terminal of switch S_(2B) arecoupled to a second output terminal.
 21. The DC-DC converter of claim20, further comprising an inductor T_(1D), wherein a first terminal ofinductor T_(1D) is coupled to the first terminal of inductor T_(1C), anda second terminal of inductor T_(1D) is coupled to the second terminalof inductor T_(1C).
 22. The DC-DC converter of claim 20, furthercomprising a capacitor C1 and a capacitor C3; wherein: a first terminalof capacitor C₁ is coupled to the first terminal of inductor T_(1A) anda second terminal of capacitor C1 is coupled to the second outputterminal; and a first terminal of capacitor C₃ is coupled to the secondterminal of inductor T_(1C) and a second terminal of capacitor C₃ iscoupled to the second output terminal.
 23. The DC-DC converter of claim20, wherein the first output terminal is coupled to a load.
 24. TheDC-DC converter of claim 20, further comprising a commutation matrixcoupled to the second terminal of inductor T_(1A), the first terminal ofinductor T_(1C), and the second output terminal.
 25. The DC-DC converterof claim 20, further comprising a controller configured to provide asignal to switch S_(1SB).